Preheat oven to 450 F with rack in upper third. Puree garlic with oil and 3/4 tsp salt in a blender until smooth. Toss sweet potatoes with garlic oil in a large bowl, then spread in 1 layer in a 15 by 10" shallow baking pan. Bake until golden in patches and cooked through, 20 to 30 minutes. To fry sage leaves, heat oil in a small heavy skillet over medium-high heat until it shimmers, then fry sage leaves in 2 batches, stirring, until crisp, 30 seconds to 1 minute per batch. Transfer with a slotted spoon to paper towels to drain. Serve sweet potatoes with sage leaves scattered on top.